
	var canvas = document.getElementById('tetris'),  //获取canvas元素
             context = canvas.getContext('2d'),  //获取画图环境，指明为2d
             centerX = canvas.width/2,   //Canvas中心点x轴坐标
             centerY = canvas.height/2,  //Canvas中心点y轴坐标
             rad = Math.PI*2/100, //将360度分成100份，那么每一份就是rad度
             speed = 0.1; //加载的快慢就靠它了
             
         //绘制蓝色外圈
         function blueCircle(n){
            context.save();
            context.strokeStyle = "#49f"; //设置描边样式
            context.lineWidth = 5; //设置线宽
            context.beginPath(); //路径开始
            context.arc(centerX, centerY, 100 , -Math.PI/2, -Math.PI/2 +n*rad, false); //用于绘制圆弧context.arc(x坐标，y坐标，半径，起始角度，终止角度，顺时针/逆时针)
            context.stroke(); //绘制
            context.closePath(); //路径结束
            context.restore();
         }
         //绘制白色外圈
         function whiteCircle(){
             context.save();
             context.beginPath();
             context.strokeStyle = "white";
             context.arc(centerX, centerY, 100 , 0, Math.PI*2, false);
             context.stroke();
             context.closePath();
             context.restore();
         } 
         
         //百分比文字绘制
         function text(n){
            context.save(); //save和restore可以保证样式属性只运用于该段canvas元素
            context.strokeStyle = "#49f"; //设置描边样式
            context.font = "40px Arial"; //设置字体大小和字体
            //绘制字体，并且指定位置
            context.strokeText(n.toFixed(0)+"%", centerX-25, centerY+10);
            context.stroke(); //执行绘制
            context.restore();
         }
         
         //动画循环
         (function drawFrame(){
                window.requestAnimationFrame(drawFrame, canvas);
                context.clearRect(0, 0, canvas.width, canvas.height);
             
                whiteCircle();
                text(speed);
                blueCircle(speed);
                
                if(speed > 100) speed = 0;
                speed += 0.1;
            }());